1. Field of the Invention
The present invention relates to a knock sensing apparatus used in a knock control system for an internal combustion engine, and more particularly to a knock sensing apparatus capable of detecting a failure therein.
2. Description of the Related Art
A method of detecting a failure in a knock sensing apparatus for an internal combustion engine is disclosed in Japanese patent application JP-A-56-66,712. In this method, a knock detecting period and a failure detecting period are separated from each other, and a failure in the knock sensing apparatus is judged on the basis of whether the output of the knock sensor in the knock sensing period is high or low. Further, another Japanese patent application JP-A-61-215,942 discloses a knock sensing apparatus for an internal combustion engine, in which a plurality of knock sensor signals (that is, output signals of a plurality of knock sensors) are successively selected by a multiplexer, and the selected knock sensor signal is applied to a band pass filter.
Even in a case where one of the knock sensors is defective, the output signal of another knock sensor passes through the band pass filter, and thus the output of the band pass filter is not reduced to zero for a predetermined time t, as shown in FIG. 1. That is, in spite of the fact that one knock sensor is defective, the output of another knock sensor is delivered in the failure detecting period of the former knock sensor, and thus the failure in the former knock sensor cannot be detected.
Further, in a case where the pass band of the band pass filter is changed when one of cylinders is selected or when engine conditions are greatly altered, there arises a problem owing to the delay of the signal generated immediately after the change of the pass band and noise caused by the change of the pass band, such that it becomes impossible to detect the knock sensor failure correctly.